Overhead substrate handling and storage system

ABSTRACT

A method for operating a material handling system including an overhead rack defining a plurality of storage positions, first and second side rails disposed above the overhead rack, a first cross rail movably coupled to the first and second side rails, and a first transport vehicle movably coupled to the first cross rail includes positioning the first transport vehicle above at least one interior window defined in the overhead rack. At least a portion of the first transport vehicle is descended through the interior window to interface with a first load port of a first tool disposed below the overhead rack. The first transport vehicle is positioned above at least one periphery window defined in the overhead rack. At least a portion of the first transport vehicle is descended through the periphery window to interface with a second load port of a second tool disposed below the overhead rack.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

BACKGROUND

The disclosed subject matter relates generally to semiconductormanufacturing and, more particularly, to an overhead substrate handlingand storage system.

Growing technological requirements and the worldwide acceptance ofsophisticated electronic devices have created an unprecedented demandfor large-scale, complex, integrated circuits. Competition in thesemiconductor industry requires that products be designed, manufactured,and marketed in the most efficient manner possible. This requiresimprovements in fabrication technology to keep pace with the rapidimprovements in the electronics industry. Meeting these demands spawnsmany technological advances in materials and processing equipment andsignificantly increases the number of integrated circuit designs. Theseimprovements also require effective utilization of computing resourcesand other highly sophisticated equipment to aid, not only design andfabrication, but also the scheduling, control, and automation of themanufacturing process.

Turning first to fabrication, integrated circuits, or microchips, aremanufactured from modern semiconductor devices containing numerousstructures or features, typically the size of a few micrometers or less.The features are placed in localized areas of a semiconductingsubstrate, and are either conductive, non-conductive, or semi-conductive(i.e., rendered conductive in defined areas with dopants). Thefabrication process generally involves processing a number of wafersthrough a series of fabrication tools. Each fabrication tool performsone or more of four basic operations discussed more fully below. Thefour basic operations are performed in accordance with an overallprocess to finally produce the finished semiconductor devices.

Integrated circuits are manufactured from wafers of a semiconductingsubstrate material. Layers of materials are added, removed, and/ortreated during fabrication to create the integrated, electrical circuitsthat make up the device. The fabrication essentially comprises thefollowing four basic operations:

-   -   layering, or adding thin layers of various materials to a wafer        from which a semiconductor is produced;    -   patterning, or removing selected portions of added layers;    -   doping, or placing specific amounts of dopants in selected        portions of the wafer through openings in the added layers; and    -   heat treating, or heating and cooling the materials to produce        desired effects in the processed wafer.

Although there are only four basic operations, they can be combined inhundreds of different ways, depending upon the particular fabricationprocess.

To facilitate processing of wafers through a process flow, wafers aretypically grouped into lots. Each lot is housed in a common wafercarrier. Carriers are transported to various process and metrology toolsthroughout the fabrication facility to allow the required processes tobe completed to fabricate integrated circuit devices on the wafers.

Modern wafer fabrication facilities employ automated material movementsystems to satisfy ergonomic concerns and to maintain a high level ofautomation. Interbay/intrabay vehicle automated material handlingsystems may be employed to automate the transfer of wafers to the toolsrequired in the process flow. One factor contributing to the efficiencyof the material handling system is the delivery time between tools.Delivery time may vary depending on the distance between tools, thecongestion of the tools, and the distance an idle material handlingvehicle needs to travel to pick up a waiting wafer carrier. Deliverytimes directly affect tool utilization and system throughput.

Due to the large number of substrates being fabricated concurrently, alarge number of wafer carriers may be disposed in wafer storage areas,referred to as stockers, while they await further processing. Theautomated material handling system coordinates transfer of the carriersto and from the storage locations and between the various processing andmetrology tools. Moves to and from storage interrupt the process flow ofthe substrates and also add to material handling system congestion anddelay.

This section of this document is intended to introduce various aspectsof art that may be related to various aspects of the disclosed subjectmatter described and/or claimed below. This section provides backgroundinformation to facilitate a better understanding of the various aspectsof the disclosed subject matter. It should be understood that thestatements in this section of this document are to be read in thislight, and not as admissions of prior art. The disclosed subject matteris directed to overcoming, or at least reducing the effects of, one ormore of the problems set forth above.

BRIEF SUMMARY

The following presents a simplified summary of the disclosed subjectmatter in order to provide a basic understanding of some aspects of thedisclosed subject matter. This summary is not an exhaustive overview ofthe disclosed subject matter. It is not intended to identify key orcritical elements of the disclosed subject matter or to delineate thescope of the disclosed subject matter. Its sole purpose is to presentsome concepts in a simplified form as a prelude to the more detaileddescription that is discussed later.

One aspect of the disclosed subject matter is seen in a method foroperating a material handling system including an overhead rack defininga plurality of storage positions, first and second side rails disposedabove the overhead rack, a first cross rail movably coupled to the firstand second side rails, and a first transport vehicle movably coupled tothe first cross rail, including positioning the first cross rail and thefirst transport vehicle above at least one interior window defined inthe overhead rack. At least a portion of the first transport vehicle isdescended through the at least one interior window to interface with afirst load port of a first tool disposed below the overhead rack. Thefirst cross rail and the first transport vehicle are positioned above atleast one periphery window defined in the overhead rack. At least aportion of the first transport vehicle is descended through the at leastone periphery window to interface with a second load port of a secondtool disposed below the overhead rack.

Another aspect of the disclosed subject matter is seen in a method foroperating a material handling system including an overhead rack defininga plurality of storage positions, first and second side rails disposedabove the overhead rack, a first cross rail movably coupled to the firstand second side rails, a first transport vehicle movably coupled to thefirst cross rail, a first overhead rail disposed outside of the overheadrack parallel to the first and second side rails, and a second transportvehicle movably coupled to the first overhead rail. The method includespositioning the first cross rail and the first transport vehicle aboveat least one interior window defined in the overhead rack. At least aportion of the first transport vehicle is descended through the at leastone interior window. At least a portion of the second transport vehicleis descended below the overhead rack to access a load port of a firsttool disposed below the overhead rack.

Yet another aspect of the disclosed subject matter is seen in a methodfor operating a material handling system including an overhead rackdefining a plurality of storage positions, wherein the overhead rackdefines at least one interior window devoid of storage locations and atleast one periphery window along an edge of the overhead rack, first andsecond side rails disposed above the overhead rack, a first cross railmovably coupled to the first and second side rails to allow movement ofthe first cross rail in a first direction along the first and secondside rails, a first transport vehicle movably coupled to the first crossrail and operable to move in a second direction perpendicular to thefirst direction along the first cross rail, a first overhead raildisposed outside of the overhead rack parallel to the first and secondside rails, and a second transport vehicle movably coupled to the firstoverhead rail. The method includes positioning the first cross rail andthe first transport vehicle at a first position above at least oneinterior window defined in the overhead rack. The first transportvehicle is descended below the overhead rack through the interiorwindow. The first cross rail and the first transport vehicle arepositioned at a second position above at least one periphery windowdefined in the overhead rack. The first transport vehicle is descendedbelow the overhead rack through the periphery window. The secondtransport vehicle is descended below the overhead rack to access a loadport of a tool disposed below the overhead rack. An input/output portposition defined in the overhead rack is accessed with the first andsecond transport vehicles.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosed subject matter will hereafter be described with referenceto the accompanying drawings, wherein like reference numerals denotelike elements, and:

FIGS. 1-3 are isometric views of a matric material handling system;

FIG. 4 is a top view of the matrix material handling system of FIG. 1;

FIG. 5 is a top view of a matrix material handling vehicle in the systemof FIGS. 1-5;

FIG. 6 is a diagram of a purge nest for controlling the environment of awafer pod in the matrix material handling system of FIGS. 1-5; and

FIG. 7 is a cut-away side view of the matrix material handling system ofFIGS. 1-5.

While the disclosed subject matter is susceptible to variousmodifications and alternative forms, specific embodiments thereof havebeen shown by way of example in the drawings and are herein described indetail. It should be understood, however, that the description herein ofspecific embodiments is not intended to limit the disclosed subjectmatter to the particular forms disclosed, but on the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the disclosed subject matter asdefined by the appended claims.

DETAILED DESCRIPTION

One or more specific embodiments of the disclosed subject matter will bedescribed below. It is specifically intended that the disclosed subjectmatter not be limited to the embodiments and illustrations containedherein, but include modified forms of those embodiments includingportions of the embodiments and combinations of elements of differentembodiments as come within the scope of the following claims. It shouldbe appreciated that in the development of any such actualimplementation, as in any engineering or design project, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness related constraints, which may vary from one implementation toanother. Moreover, it should be appreciated that such a developmenteffort might be complex and time consuming, but would nevertheless be aroutine undertaking of design, fabrication, and manufacture for those ofordinary skill having the benefit of this disclosure. Nothing in thisapplication is considered critical or essential to the disclosed subjectmatter unless explicitly indicated as being “critical” or “essential.”

The disclosed subject matter will now be described with reference to theattached figures. Various structures, systems and devices areschematically depicted in the drawings for purposes of explanation onlyand so as to not obscure the disclosed subject matter with details thatare well known to those skilled in the art. Nevertheless, the attacheddrawings are included to describe and explain illustrative examples ofthe disclosed subject matter. The words and phrases used herein shouldbe understood and interpreted to have a meaning consistent with theunderstanding of those words and phrases by those skilled in therelevant art. No special definition of a term or phrase, i.e., adefinition that is different from the ordinary and customary meaning asunderstood by those skilled in the art, is intended to be implied byconsistent usage of the term or phrase herein. To the extent that a termor phrase is intended to have a special meaning, i.e., a meaning otherthan that understood by skilled artisans, such a special definition willbe expressly set forth in the specification in a definitional mannerthat directly and unequivocally provides the special definition for theterm or phrase.

Referring now to the drawings wherein like reference numbers correspondto similar components throughout the several views and, specifically,referring to FIGS. 1-5, the disclosed subject matter shall be describedin the context of a matrix material handling system (MMHS) 100. FIGS.1-3 are various isometric views of the MMHS 100, and FIG. 4 is a topview of the MMHS 100. The MMHS 100 is disposed over a plurality ofmanufacturing tools 110, such as tools used in the fabrication ofsemiconductor devices. In a semiconductor fabrication environment,exemplary manufacturing tools 110 include processing tools (e.g.,photolithography steppers, etch tools, deposition tools, polishingtools, rapid thermal processing tools, implantation tools, etc.),metrology tools, sorters, etc.

The particular tools 110 disposed below the MMHS 100, and theirarrangement may vary depending on the particular implementation and theprocessing steps being performed. In one example, tools 110 in a commontool family may be grouped in common control areas. Hence,photolithography tools may be located in one control area, while etchtools may be located in another control area. In another example, thetools 110 may be grouped by process layer. Hence, the tools required toform a particular layer (i.e., starting with a photolithography step andterminating prior to the next photolithography step) may be grouped intoa common control area.

The MMHS 100 includes one or more linear material handling vehicles 120and one or more matrix material handling vehicles 130. Generally, thelinear material handling vehicles 120 move along overhead rails 140disposed in aisles 150 between the tools 110. An overhead rack 160defines a plurality of storage positions 170 over the tools 110 forreceiving wafer pods 180. The linear material handling vehicles 120 movewafer pods 180 between different areas of a manufacturing facility, toone of the tools 110, or to one of the storage positions 170 in theoverhead rack 160. For example, predefined input/output (I/O) portpositions 190 may be defined along the periphery of the overhead rack160 to receive or dispatch pods 180 from or to the overhead rack 160. Inone embodiment, an I/O port 190 may be provided on each side of theoverhead rack 160.

The matrix material handling vehicles 130 move pods 180 to variouspositions within the overhead rack 160 or to one of the tools 110. Thematrix material handling vehicles 130 are movably coupled to a gantrydrive system including side rails 200 and a cross rail 210, as shown inFIG. 5. The cross rail 210 includes a drive mechanism for moving alongthe side rails 200, and the matrix material handling vehicles 130include a drive mechanism for moving along the cross rail 200 to accessthe various storage positions 170. The linear material handling vehicles120 and the matrix material handling vehicles 130 also include hoistsystems for raising or lowering the pods 180 to engage the overhead rack160 or to interface with a load port 220 of one of the tools 110.

Drive systems for moving the vehicles 120, 130 along the rails 140, 200,210 and hoist systems for raising and lowering the pods 180 to interfacewith the overhead rack 160 or the tools 110 are known to those ofordinary skill in the art, so they are not described in greater detailherein to avoid obscuring the present subject matter.

Certain storage positions 181 may be equipped with equipment toestablish a vacuum and/or to provide nitrogen gas, extremely clean dryair (XCDA), or some other purge gas) for pods 180 stored therein. Thesepods 180 may be stored under protected conditions (e.g., to avoidoxidizing exposed regions of the wafers) near the tools 110 needed forthe next process operation. This protected storage near the tool 110increases throughput and yield. An exemplary storage location 181equipped with a purge nest 182 is shown in FIG. 6. The purge nest 182includes a frame 183 for supporting a wafer pod 180. A purge port 184fed by a gas supply line 185 is provided to supply a cover gas for theinterfacing pod 180 (not shown). A vacuum port 186 coupled to a vacuumline 187 may be used to remove the purge gas exiting the pod 180.

The overhead rack 160 defines one or more interior windows 230 to allowa matrix material handling vehicle 130 to interface with a load port 240of a tool not disposed along the periphery of the overhead rack 160(i.e., along an aisle 150). The matrix material handling vehicle 130 maybe provided with rotating grippers to allow a wafer pod 180 to berotated as well as lowered, so that the pod 180 may be aligned at anyangle (e.g., aligned with various cluster tool facets). The overheadrack 160 also defines periphery windows 250 to allow access to theaisle-oriented load ports 220.

The overhead rack 160 may be constructed of a plurality interlockinggrid pieces that can be dynamically configured to arrange the windows230 relative the load ports 240. For tools 110 that are susceptible toparticulate contamination (e.g., while they are opened duringpreventative maintenance procedures), a fan filter unit (FFU) containinga high efficiency particulate air (HEPA) filter may be mountedimmediately beneath the overhead rack 160. For tools 110 that haveutilities or exhaust ducts passing through the ceiling, the utilitiesand exhaust may be grouped to penetrate an interior window 230selectively placed in the matrix, or grouped adjacent to the overheadrack 160 so as to not inhibit the travel of the matrix material handlingvehicles 130 over tool load ports 220.

Either the linear material handling vehicles 120 or the matrix materialhandling vehicles 130 can access the aisle-oriented load ports 220 toload the tools 110. Generally, a linear material handling vehicle 120lowers the pod 180 and reaches out to engage the load port 220, whilethe matrix material handling vehicle 130 traverses through the peripherywindow 250 to engage the pod 180 with the load port 220.

FIG. 7 illustrates a cut-away side view of the MMHS 100 illustrating howtool density may be increased due to the overhead and matrix vehicles120, 130. In the embodiment illustrated in FIG. 7, the system 100includes linear material handling vehicles 120A-F and matrix materialhandling vehicles 130A-B. The inside linear material handling vehicles120C, 120D may be provided to allow traffic to bypass the illustratedportion of the MMHS 100. The linear material handling vehicles 120A,120B, 120E, 120F may be used to load tools 110A-E or to transfer pods180 in to and out of their respective portions of the overhead rack160A, 160B. The tools 110A-E may be arranged with load ports 220A-D thatare disposed on edges of the MMHS 100 and load ports 240A, 240B that aredisposed not on the edges. Interior windows 235A-B and periphery windows250A-D are provided to allow the matrix material handling vehicles130A-B to access the various load ports 220A-D, 240A-B.

For example, the load port 220A disposed along the edge may be accessedby the linear material handling vehicle 120A or by the matrix materialhandling vehicle 130A through the periphery window 250A. The load port240A that is not disposed along the edge may be accessed by the matrixmaterial handling vehicle 130A through the interior window 235A. Thelayout of the tools 110A-E may be varied depending on the amount ofavailable floor space and the size and port positions of the tools toimprove the density of the layout.

Because the matrix material handling vehicle 130 can interface with atool 110 through an interior window 230, the tools 110 need not bearranged in a completely linear fashion, as is the case in aconventional machine layout. Because the size and port orientation ofthe various tools 110 may vary, avoiding a linear layout allows a densertool layout, thereby conserving floor plan space to increase fabcapacity and reducing the traversal distance between tools 110 toincrease throughput. Due to the number of storage positions 170 in theoverhead rack 160 conventional stockers need not be provided in the MMHS100, thereby reducing overall system cost and increasing throughput byavoiding moves to and from the stockers.

In one embodiment, the tools 110 disposed along the aisles 150 may beprovided with conventional SEMI ports 240 for receiving conventionalfront opening unified pods (FOUP). These conventional ports 240 may beaccessed by either the linear material handling vehicles 120 or thematrix material handling vehicles 130. Tools 110 disposed near theinterior windows 230 may be provided with advanced ports for receivingadvanced wafer pods. For example, pods 180 may be provided that do notopen to external atmosphere for loading or unloading. A protective gasmay be provided during the transfer operation. The advanced load portmay be provided for a cluster tool 110, a carrier capable of directlyinterfacing with a vacuum, etc. The use of advanced pods allows directprocess to process moves, which increased both yield and throughput.These direct moves also eliminates the need for FOUP handling steps,thereby reducing hardware requirements and improving cycle times.

The overhead rack 160 may be shared by more than one matrix materialhandling vehicle 130. For example, as shown in FIG. 2, four or morecross rails 200 may be provided over the rack 160, each with its ownmatrix material handling vehicle 130. Shared regions may be defined inthe overhead rack 160 that can be accessed by different matrix materialhandling vehicles 130. One matrix material handling vehicle 130 canplace a pod 180 in a storage position 170 after processing by a tool110, and another matrix material handling vehicle 130 can retrieve thepod 180 at a later time to move it to a different tool 110 for the nextoperation. If one matrix material handling vehicle 130 fails, anothermatrix material handling vehicle 130 can bump the cross rail 200 out ofthe way to access storage positions 170 in the overhead rack 160 thathad been serviced by the failed matrix material handling vehicle 130.

The MMHS 100 eliminates single points of failures because the overheadrack 160 can be loaded from by the linear material handling vehicles 120using overhead rails 140 on either side. In cases where there is nofailure, this effectively doubles the throughout density. Overlappingportions of the overhead rack 160 may be accessed by different matrixmaterial handling vehicle 130. The two-dimensional capabilities of thematrix material handling vehicles 130 also allow fast swapping at thetools 110 and access to tools 110 disposed beneath the overhead rack160. Traffic blockages associated with conventional linear materialhandling systems may be avoided due to the increased number of movementaxes.

The proximity of the overhead rack 160 to the tools 110 allows sharedlocal buffering for tools 110 of the same type. Multiple pods 180requiring the same operation may be stored proximate tools 110 of thesame type without requiring the scheduling system to identify theparticular tool 110 that will perform the next operation. The matrixmaterial handling vehicles 130 may deliver the pod 180 to the selectedtool 110 after the dispatch decision is made without incurring amaterial handling delay. Kits of test wafers may also be storedproximate to tools 110 where they may be employed (e.g., to qualify atool after maintenance) to save cycle time and reduce material handlingtraffic.

Scheduling for the MMHS 100 may be provided by centralized and localschedulers. A centralized scheduler schedules global moves within thesystem 100, while local controllers control moves for pods 180 stored onthe overhead rack 160 for a group of tools 110 to effect the processingof the wafers over a plurality of process steps. An exemplary schedulingsystem is described in U.S. patent application Ser. No. 13/247,792,entitled “Methods and Systems for Semiconductor Fabrication with LocalProcessing Management”, and incorporated herein by reference in itsentirety.

The particular embodiments disclosed above are illustrative only, as thedisclosed subject matter may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular embodiments disclosed above may be altered or modified andall such variations are considered within the scope and spirit of thedisclosed subject matter. Accordingly, the protection sought herein isas set forth in the claims below.

We claim:
 1. A method for operating a material handling system includingan overhead rack defining a plurality of storage positions, first andsecond side rails disposed above said overhead rack, a first cross railmovably coupled to said first and second side rails, and a firsttransport vehicle movably coupled to said first cross rail, comprising:positioning said first cross rail and said first transport vehicle aboveat least one interior window defined in said overhead rack; descendingat least a portion of said first transport vehicle through said at leastone interior window to interface with a first load port of a first tooldisposed below said overhead rack; positioning said first cross rail andsaid first transport vehicle above at least one periphery window definedin said overhead rack; and descending at least a portion of said firsttransport vehicle through said at least one periphery window tointerface with a second load port of a second tool disposed below saidoverhead rack.
 2. The method of claim 1, further comprising interfacinga pod coupled to said first transport vehicle with said first load port.3. The method of claim 2, further comprising rotating said pod usingsaid first transport vehicle to align said pod with said first loadport.
 4. The method of claim 1, further comprising interfacing a podcoupled to said first transport vehicle with said second load port. 5.The method of claim 1, wherein said material handling system includes afirst overhead rail disposed along a first edge of said overhead rackparallel to said first and second side rails, a second transport vehiclemovably coupled to said first overhead rail, a second overhead raildisposed along a second edge of said overhead rack parallel to saidfirst and second side rails, and a third transport vehicle movablycoupled to said first overhead rail, and the method further comprises:positioning said second transport vehicle to engage a first pod disposedat a first input/output port position defined in said overhead rackalong said first edge; and positioning said third transport vehicle toengage a second pod disposed at a second input/output port positiondefined in said overhead rack along said second edge.
 6. The method ofclaim 1, wherein said material handling system includes a second crossrail movably coupled to said first and second side rails and a secondtransport vehicle movably coupled to said second cross rail, and themethod further comprises engaging said first cross rail with said secondcross rail to push said first cross rail along said first and secondcross rails.
 7. The method of claim 1, wherein said material handlingsystem includes a purge nest disposed on said overhead rack and havingat least one gas port, and the method further comprises engaging saidgas port with a pod supported by said overhead rack.
 8. The method ofclaim 7, wherein engaging said gas port further comprises providing apurge gas to said pod.
 9. A method for operating a material handlingsystem including an overhead rack defining a plurality of storagepositions, first and second side rails disposed above said overheadrack, a first cross rail movably coupled to said first and second siderails, a first transport vehicle movably coupled to said first crossrail, a first overhead rail disposed outside of said overhead rackparallel to said first and second side rails, and a second transportvehicle movably coupled to said first overhead rail, comprising:positioning said first cross rail and said first transport vehicle aboveat least one interior window defined in said overhead rack; descendingat least a portion of said first transport vehicle through said at leastone interior window; and descending at least a portion of said secondtransport vehicle below said overhead rack to access a load port of afirst tool disposed below said overhead rack.
 10. The method of claim 9,further comprising descending at least a portion of said first transportvehicle through said at least one interior window defined in saidoverhead rack to interface with a second load port of a second tooldisposed below said overhead rack.
 11. The method of claim 10, furthercomprising descending at least a portion of said first transport vehiclethrough at least one periphery window defined in said overhead rack tointerface with a third load port of a third tool disposed below saidoverhead rack
 12. The method of claim 10, further comprising interfacinga pod coupled to said first transport vehicle with said second loadport.
 13. The method of claim 12, further comprising rotating said podusing said first transport vehicle to align said pod with said secondload port.
 14. The method of claim 11, further comprising interfacing apod coupled to said first transport vehicle with said third load port.15. The method of claim 9, wherein said material handling systemincludes a first overhead rail disposed along a first edge of saidoverhead rack parallel to said first and second side rails, a thirdtransport vehicle movably coupled to said first overhead rail, a secondoverhead rail disposed along a second edge of said overhead rackparallel to said first and second side rails, and a fourth transportvehicle movably coupled to said first overhead rail, and the methodfurther comprises: positioning said third transport vehicle to engage afirst pod disposed at a first input/output port position defined in saidoverhead rack along said first edge; and positioning said fourthtransport vehicle to engage a second pod disposed at a secondinput/output port position defined in said overhead rack along saidsecond edge.
 16. The method of claim 9, wherein said material handlingsystem includes a second cross rail movably coupled to said first andsecond side rails and a third transport vehicle movably coupled to saidsecond cross rail, and the method further comprises engaging said firstcross rail with said second cross rail to push said first cross railalong said first and second cross rails.
 17. The method of claim 9,wherein said material handling system includes a purge nest disposed onsaid overhead rack and having at least one gas port, and the methodfurther comprises engaging said gas port with a pod supported by saidoverhead rack.
 18. The method of claim 17, wherein engaging said gasport further comprises providing a purge gas to said pod.
 19. A methodfor operating a material handling system including an overhead rackdefining a plurality of storage positions, wherein said overhead rackdefines at least one interior window devoid of storage locations and atleast one periphery window along an edge of said overhead rack, firstand second side rails disposed above said overhead rack, a first crossrail movably coupled to said first and second side rails to allowmovement of said first cross rail in a first direction along said firstand second side rails, a first transport vehicle movably coupled to saidfirst cross rail and operable to move in a second directionperpendicular to said first direction along said first cross rail, afirst overhead rail disposed outside of said overhead rack parallel tosaid first and second side rails, and a second transport vehicle movablycoupled to said first overhead rail, comprising: positioning said firstcross rail and said first transport vehicle at a first position above atleast one interior window defined in said overhead rack; descending saidfirst transport vehicle below said overhead rack through said interiorwindow; positioning said first cross rail and said first transportvehicle at a second position above at least one periphery window definedin said overhead rack; descending said first transport vehicle belowsaid overhead rack through said periphery window; descending said secondtransport vehicle below said overhead rack to access a load port of atool disposed below said overhead rack; and accessing an input/outputport position defined in said overhead rack with said first and secondtransport vehicles.